U.S. patent number 4,731,570 [Application Number 06/943,213] was granted by the patent office on 1988-03-15 for electrical drive circuit for a variable-speed switched reluctance motor.
This patent grant is currently assigned to Caterpillar Inc.. Invention is credited to Peter W. Lee.
United States Patent |
4,731,570 |
Lee |
March 15, 1988 |
Electrical drive circuit for a variable-speed switched reluctance
motor
Abstract
An electrical drive circuit for a variable-speed switched
reluctance motor having a bifilar winding is provided. First,
second, and third thyristor switches (5,7,10) each having
respective firing circuits (6,8,11) are associated with the motor.
A commutation capacitor device (9) is associated with one of the
thyristor switches (5,7,10). A sensing means (33) determines the
direction of current flow through the commutation capacitor device
(9), and a control system (20) prevents the first and third
thyristor switches (5,10) from conducting when the second thyristor
switch (7) is conducting and prevents the first and second
thyristor switches (5,7) from conducting when the third thyristor
switch (10) is conducting.
Inventors: |
Lee; Peter W. (Willingham,
GB2) |
Assignee: |
Caterpillar Inc. (Peoria,
IL)
|
Family
ID: |
10584918 |
Appl.
No.: |
06/943,213 |
Filed: |
September 8, 1986 |
PCT
Filed: |
September 08, 1986 |
PCT No.: |
PCT/US86/01846 |
371
Date: |
September 08, 1986 |
102(e)
Date: |
September 08, 1986 |
PCT
Pub. No.: |
WO87/01530 |
PCT
Pub. Date: |
March 12, 1987 |
Foreign Application Priority Data
Current U.S.
Class: |
318/696; 318/685;
318/701 |
Current CPC
Class: |
H02P
25/0925 (20160201) |
Current International
Class: |
H02P
25/02 (20060101); H02P 25/08 (20060101); H02P
008/00 () |
Field of
Search: |
;318/696,685,701,138,258 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shoop, Jr.; William M.
Assistant Examiner: Bergmann; Saul M.
Attorney, Agent or Firm: Noe; Stephen L.
Claims
I claim:
1. An electrical drive circuit for a variable-speed switched
reluctance motor for connection to a DC power source, and said
motor having a bifilar winding, said bifilar winding having a main
winding (3) and an auxiliary winding (3'), comprising:
first and second voltage/current lines (1,2) connectable to said DC
power source, one end of said main winding (3) being connected to
said first voltage/current line (1);
first, second and third thyristor switches (5,7,10) each having a
respective firing circuit (6,8,11) adapted to fire said respective
thyristor switch (5,7,10) to conduction, said first thyristor
switch (5) being connected between said second end of said main
winding (3) and said second voltage/current line (2), said second
and third thyristor switches (7,10) being connected in series
between said second end of said main winding (3) and said second
voltage/current line (2), in parallel with said first thyristor
switch (5);
a commutation capacitor device (9), said commutation capacitor
device (9) being connected in parallel with said third thyristor
switch (10) between said second and third thyristor switches (7,10)
and said second voltage/current line (2);
means (33) for sensing the direction of current flow through said
commutation capacitor device (9); and
control means (20) for preventing said first and third thyristor
switches (5,10) from conducting when said second thyristor switch
(7) is conducting and to prevent said first and second thyristor
switches (5,7) from conducting when said third thyristor switch
(10) is conducting, said control means (20) being connected to said
sensing means (33) and to said firing circuits (5,8,11).
2. A circuit, according to claim 1, wherein said means (33) for
sensing said direction of current flow through said commutation
capacitor (9) device comprises a Hall effect sensor.
3. A circuit, according to claim 2, wherein said control means (20)
includes means (17) for monitoring the current through said first
thyristor switch element (5).
4. A circuit, according to claim 3, wherein said monitoring means
(17) includes a Hall effect device (33).
5. A circuit, according to claim 3, wherein said control means (20)
further includes means for monitoring the voltage across said
commutation capacitor device (9).
6. A circuit, according to claim 5, wherein said commutation
capacitor device voltage and said current through said first
thyristor switch element (5) are delivered to a comparator, said
comparator having an output adapted to control the firing of said
second thyristor switch element (7), said second thyristor switch
element (7) being fired when the value of current through said
first thyristor switch element (5) is at a maximum value relative
to the set value of said commutation capacitor negative voltage,
whereby said first thyristor switch element (5) is force commutated
off.
Description
DESCRIPTION
1. Technical Field
The present invention relates to electrical drive circuits for
motors and, more particularly, to a drive circuit for a
variable-speed switched reluctance motor having a bifilar winding,
that is to say main and auxiliary windings integrally wound on the
motor.
2. Background Art
In recent years considerable interest has been shown in the use of
switched-reluctance motor drives as a replacement for conventional
induction motor drives. The former have been shown to be able to
achieve unusually good combinations of high power output and system
efficiency while having costs considerably below those of AC-motor
systems. Variable-speed switched reluctance machines are also often
able to provide a range and quality of control usually only
associated with the best DC-motor systems, while providing a range
of substantial operational advantages in terms of reliability and
robustness.
Various drive circuits are known and, in particular, drive circuits
using thyristor switches have been utilized to provide the desired
commutation control of the individual phases of the motor. Each
phase has its own respective drive circuit and these are identical
with one another. Conventional circuits utilize a commutation
capacitor which is initially charged to a supply voltage by the
firing of a first thyristor switch which is then switched off, a
second thyristor switch being closed to allow current produced by
the capacitor to flow through the load winding of the motor. As
there are advantages in using bifilar wound motors, modified
circuits have been utilized, but those studied have suffered from
various disadvantages.
The present invention is directed to overcoming one or more of the
problems as set forth above.
DISCLOSURE OF THE INVENTION
According to the present invention, an electrical drive circuit for
a variable-speed switched reluctance motor connectable to a DC
power source is provided. The motor has a bifilar winding having a
main winding and an auxiliary winding. The drive circuit includes
first and second voltage/current lines for connection to the DC
power source, one end of the main winding being connected to the
first voltage/current line. A resonant reversal inductor is also
included.
First, second and third thyristor switches each have a respective
firing circuit for firing the thyristor switches to conduction. The
first thyristor switch is connected between the second end of the
main winding and the second voltage/current line. The second and
third thyristor switches are connected in series with the inductor
between the second end of the main winding and the second
voltage/current line, in parallel with the first thyristor switch.
A commutation/snubber capacitor device is connected in parallel
with the third thyristor switch between the second thyristor switch
and the second voltage/current line. Means are included for sensing
the direction of current flow through the commutation/snubber
capacitor device, thereby to sense current flow through either the
second or the third thyristor switches.
Finally, control means are provided for preventing the first and
third thyristor switches from conducting when the second thyristor
switch is conducting and to prevent the first and second thyristor
switches from conducting when the third thyristor switch is
conducting, the control means being connected to the sensing means
and to the firing circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
One example of one phase of a drive circuit according to the
present invention will now be described with reference to the
accompanying drawings in which:
FIG. 1 shows the basic arrangement of a drive circuit for one phase
(in a 3-phase machine there will be two further identical
circuits);
FIG. 2 illustrates a first thyristor firing circuit;
FIG. 3 illustrates a second thyristor firing circuit;
FIG. 4 illustrates a current sensing circuit;
FIG. 5 illustrates a control means for the thyristor firing
circuits; and
FIGS. 6 and 7 are timing diagrams for various signals of the
circuit.
BEST MODE FOR CARRYING OUT THE INVENTION
A battery (not shown) provides a DC power source at 72 volts to
supply current to a pair of lines 1,2. A main winding 3 and an
auxiliary winding 3' are wound in a bifilar configuration on the
motor. Between the main winding 3 and the line 2, a first thyristor
switch 5 is provided, the switch 5 being able to conduct, as shown,
in the direction from the main winding to the line 2. A commutation
diode 12 is provided in series with the auxiliary winding. A firing
circuit 6 for the thyristor switch 5 is indicated schematically in
FIG. 1 and in detail in FIG. 2. To the same end of the main winding
3 a second thyristor switch element 7 is connected, coupled between
the main winding 3 and a commutation/snubber capacitor device 9.
The thyristor switch element 7 has a firing circuit 8, shown
schematically in FIG. 1 and in detail in FIG. 3. A bypass capacitor
device 23 is also provided. A third thyristor switch element 10 is
coupled between the second thyristor switch element 7 and the line
2 in parallel with the commutation/snubber capacitor device 9 and
in series with an inductor 14. The third thyristor switch element
10 has a firing circuit 11 again shown schematically in FIG. 1 and
in detail in FIG. 3.
In operation, the second thyristor switch 7 is fired first causing
the main winding 3 and the commutation/snubber capacitor device 9
to "ring" with a half cycle sinusoidal current, the voltage across
the commutation/snubber capacitor 9 peaking nominally at twice the
battery voltage and being caused to remain at this peak voltage by
the third thyristor switch element 10 being in the forward blocking
condition.
The third thyristor switch 10 is then fired to reverse the voltage
on the capacitor device 9, to a negative potential slightly less
than its positive value (due to circuit losses), by means of the
inductor 14, so that the capacitor 9 is negatively charged and
ready for the commutation process. At this point, the noncyclic
operation terminates and the operation hereafter follows the
following cycle. The first thyristor switch 5 is fired causing load
current to flow in the main winding 3. When the current in the main
winding 3 has reached a predetermined value, then the second
thyristor switch 7 is fired so that the principal current is
diverted from the first thyristor switch element 5 which is turned
off. The anode voltage of the first thyristor switch 5 increases to
a positive value diverting current to the commutation/snubber
capacitor 9 via the second thyristor switch 7. By transformer
action between the main 3 and auxiliary 3' windings, at a value of
about twice the battery voltage, the commutation diode 12 conducts,
returning the principal current to the supply.
The anode voltage of the first thyristor switch 5 continues to rise
to a value of twice the battery voltage plus a voltage
corresponding to the energy stored in the uncoupled leakage
inductance of the machine and the supply conductors and when this
energy is completely "snubbed" the second thyristor switch 7
naturally commutates off.
The circuit is then ready for the next cycle. It will be observed
that the next cycle does not require the capacitor to be charged
via the main winding as described initially.
It will be appreciated that second and third thyristor switches 7
and 10 provide, if fired together, a short circuit path for current
between the lines 1 and 2, which is potentially damaging, and,
accordingly, it is necessary to ensure that the first and third
thyristor switches 5,10 cannot conduct when the second thyristor
switch 7 is conducting and likewise to prevent the first and second
thyristor switches 5,7 from conducting when the third thyristor
switch 10 is conducting. To this end a Hall effect sensor 13 is
provided, to supply a detected current signal (see FIG. 4) to a
control means which determines the direction of the current and in
turn provides signals inhibiting firing of the firing circuits of
the three thyristor switch elements as appropriate. The Hall effect
sensor 13 is located, in the example shown, around the conductor 16
between the second thyristor 7 and the capacitor 9 to sense current
flow therethrough.
FIG. 4 illustrates a sensing circuit 17 which includes the Hall
effect sensor 13 and a pair of bi-polar detectors 37,37' which
determine the direction of current flow in the conductor 16, thus
determining which of the second and third thyristor switches 7,10
is conducting. The outputs 18,19 are fed to the main circuit board
(FIG. 5) and then via delay circuitry 21 in the control circuit 20
to appropriate logic circuitry 22 to ensure that the firing of the
switches 5,7,10 is appropriately inhibited.
The firing conditions for the three thyristor switches 5,7,10 are
as follows:
Switch 10 fires if:
(a) Positive capacitor voltage is high;
(b) Current through switch 7 is zero; and
(c) Firing pulse is on.
Switch 7 fires primarily if:
(a) Negative capacitor voltage is low;
(b) Positive capacitor voltage is low;
(c) Current through switch 10 is zero; and
(d) Firing pulse is on.
Switch 5 fires if:
(a) Current limit is off;
(b) Negative capacitor voltage is high;
(c) Current through switch 7 is zero;
(d) Current through switch 10 is zero; and
(e) Firing pulse is on.
Operation of the motor, its speed, etc. is controlled via a
microprocessor (not shown) providing signal inputs at 15,15' in
FIG. 5, which are fed into the logic circuitry 22 to determine, in
accordance with the above, firing of the thyristor switches and
hence motor control.
It is apparent from the circuit diagram that the shunt commutation
route of the second thyristor switch element 7, tnird thyristor
switch element 10, and inductor 14 constitutes an unrecoverable
short circuit via the motor winding 3, should both devices be fired
at the same time. From the above description of the circuit
operation, it is also apparent that the charge condition of the
capacitor device 9 is a critical element in the decision chain
leading to the ultimate firing of the first thyristor switch
element 5. To supervise the firing sequence, a number of decision
elements are necessary:
1. A measurement of current flowing in the first thyristor switch
element 5 (if current is flowing).
2. A measurement of the capacitor voltage and its polarity.
3. Current detection in the second thyristor switch element 7 and
the third thyristor switch element 10.
With these elements, it is possible to interlock the power system
such that the control processor will demand current into the motor
only when the correct conditions exist within the power system.
Supervision of the firing sequence and the resulting current
amplitude is controlled within the power section. Unacceptable
demands from the control processor are ignored until the correct
conditions for the demand are available.
An OEM Hall effect current transducer 33 is used for measurement of
current in the conductor 4 and hence, in the first thyristor switch
element 5. A second loop 4' is passed through the current
transducer 33 to determine the value of the decaying current in the
auxiliary winding 3'. This loop is in the opposing direction which
allows an undirectional current signal from the current transducer
33.
The peak value of current and the decayed value are principal
control parameters for the microprocessor.
A separate power supply is used for the current transducer 4 and
the output signal is fed directly to a current detector circuit 33'
(FIG. 5).
To avoid noise and interaction between the microprocessor and the
power electronics, an analog optical coupler 23 is used to transfer
the current value to the logic PCB. FIG. 5 shows this coupling. A
voltage to current converter 24 is used to convert the 0-10 volt
transducer signal to 0-20 mA with a linear transfer
characteristic.
The input signal is scaled by adjusting a variable resistor 24' to
present 5 volts DC to pin 3 of converter 24. There are two reasons
for this scaling, firstly 10 volts is too close to the input range
of the amplifier/converter 24, and also the current limit value may
be varied by adjusting the resistor 24' during the test program. As
a final adjustment, 1000 amperes gives 5 volts DC at the output of
IC24, pin 7.
For capacitor voltage measurements, firstly a negative, analog
opto-coupler 26 is used for this function, identical to the current
measurement system. With the machine current switched off at 1000
amperes, then charge reversed, the maximum negative excursion is
-600 volts and the feed resistor (not shown) to terminal C is 30 k
. This produces a resultant current of 20 mA through the LED 26 and
10 volts at pin 7 of the comparator 28.
The comparator 28 pin 1 compares the negative voltage value with a
preset reference level. This is set such that pin 1 is high if the
actual negative voltage is greater than 100 volts DC and goes low
if the actual voltage falls below 90 volts DC. Reference will be
made to this function in the operating description.
For measuring positive capacitor voltage, a digital opto-coupler 29
is provided which has well defined transfer characteristics both in
LED operating current to switch the device on and in the hysteresis
to the off state. The current into the LED is fed by the same
resistance, 30 K, as the negative voltage system. At approximately
55 volts DC, the output of the coupler 29 goes low, when the
voltage falls to 40 volts DC, the output goes high. A delay is
introduced into the signal by an R/C network 30,31, but only during
the 1-0 transition. This delay can be reset by the transistor 32.
The reason for this delay is made clear in the operating
description.
Current detection in the second thyristor switch element 7 and the
third thyristor switch element 10 is discussed with reference to
FIG. 4.
To detect current in the second thyristor switch element 7 or the
third thyristor switch element 10, the toroidal ferrite core 13 has
the common connection from these devices passed through it on its
way to the commutation capacitor 9. A slot is cut in the toroid and
a linear Hall effect device 34 is potted in the slot. A resistance
35 is used to trim the DC offset of the device 34 to zero with no
current flowing. A differential amplifier 36 with a gain of 10
PG,11 is provided. In the common cable the second thyristor switch
element current is positive and the third thyristor switch element
current is negative--hence dual supply rails are used for the
amplifier 36. Two simple comparators 37,37' are connected to the
differential amplifier 35 output, one 37 with a positive threshold
and one 37' negative. A single polarity signal is available at 18
and 19 for transmission to the logic board (FIG. 5).
Each signal goes low for "current present" with a minimum
resolution of 10 amperes. A reduced resolution can be obtained by
increasing the threshold level potentiometers 38,39.
Industrial Applicability
Preferably, current flow through the commutation/snubber capacitor
device is sensed by means of a Hall effect sensor located around a
conductor which extends from the commutation/snubber capacitor
device to a point between the second and third thyristor switches.
As current flow through the second thyristor switch 7 causes
current flow through the said conductor in a direction opposite to
the direction of flow of current caused by current flow through the
third thyristor switch 10, the direction of current across the
commutation/snubber capacitor device 9 can be sensed so as to be
utilized by the control means to provide an interlock to prevent
the first and third thyristor switches 5,10 from conducting when
the second thyristor switch 7 is conducting and to prevent the
first and second thyristor switches 5,7 from conducting when the
third thyristor switch 10 is conducting.
It is essential to control current through the first thyristor
switch element 5 to match the commutation capacitor status. This
supervision task is the most critical of the interlock protection
system. Two necessary elements are available to ensure commutation,
i.e.,
a. The value of negative charge (V.sub.CAP).
b. A continuous monitor of the first thyristor switch element 5
current.
Referring to the circuit diagram of FIG. 5, these two circuit
elements are fed to the comparator 40. The output of the comparator
40 pin 1 is buffered, then pushed through the integrated circuit 41
direct to the second thyristor switch element 7 firing circuit.
This function is not impeded in any way by other considerations. As
soon as the maximum value of current is reached against its set
value of commutation capacitor negative voltage, the second
thyristor switch element 7 is fired which force commutates the
first thyristor switch element 5 into the off state.
At 1000 amperes the IC25 pin 7 output is 5 volts. If this current
is switched off, then the commutation capacitor voltage, when
charge reversed, will reach minus 600 volts corresponding to 10
volts at the output of the comparator 28 pin 7. This point is,
however, clamped by the diode 42 at 5 volts. By implication only
300 volts on the capacitor 9 must be capable of commutating the
1000 ampere first thyristor switch element 5 current. This is in
fact the case, and from system tolerancing a value of 250 volts is
sufficient to commutate the 1000 amperes. A scaling factor is
essential for satisfactory operation of the motor.
During operation when one phase of the motor is energized, the
current does not rise linearly with time since during rotation the
inductance is increasing. To maintain the energy expression at the
end of the defined conduction period, the current value may be
substantially less than the value at the beginning. The energy
expression is the amount of energy that must be stored in the
communication capacitor to ensure commutation of the associated
thyristor. There must therefore be a suitable ratio between the
available commutation energy from the smaller current to allow the
larger current to be safe at the beginning of the next cycle. The
1000 ampere - 250 volts scaling ensures this requirement.
In FIG. 5, inputs are at the left moving through, mostly sequential
logic, to outputs on the right. The top left-hand side is the
machine winding current input which is optically coupled via an
analog coupler configuration, 23,25,40. Next, V.sub.CAP, the
positive capacitor voltage measurement, is transferred across by a
defined operating current/hysteresis digital opto-coupler 43.
The V.sub.CAP input also feeds the negative capacitor voltage
analog coupler 29, which is of the same configuration as the
coupler 23. The inputs 18 and 19 are "current present" signals in
the third thyristor switch element 10 and the second thyristor
switch element 7. Finally, the firing signals from the processor
inputs 15 and 15' are optically coupled using a digital
opto-coupler 45.
Three outputs 63,64,65 are located on the right hand side of the
circuit diagram which are the logic output pulses to the thyristor
switch element firing circuits (FIGS. 2 and 3). These are
conventional circuits and will not be further described, but it
should be noted that the firing circuits of FIG. 3 used for the
second and third thyristors 7 and 10 are transformer-coupled to the
respective thyristors. The LED's at the firing circuit input are in
series with the output connections located at the respective IC's
46. The monostables 47,48 are connected to the nonretriggerable
mode to supply a 50 S output pulse. Before the pulse is transferred
to the output buffer, a respective AND gate 49 is inhibited by a
power up delay IC50. This AND gate group is also inhibited during
the firing of any one monostaole 47,48, i.e., the Q output of tne
fired monostable is fed to the remaining two AND gates, inhibiting
coincident firing. The primary steering gates for the output pulse
activation are 51,52,53. It can be seen that the input firing pulse
from the processor inputs 15,15' is fed to all three gates. As the
various remaining input conditions at these IC's go to the "allow"
state, then a thyristor switch element firing pulse is produced.
Overall, the system is self synchronous, that is, as the various
circuit conditions are satisfied, the next thyristor in the
sequence is fired.
At the instant of switch on with the firing input low, nothing
happens at the firing outputs. The delay IC50 power up inhibit
suppresses any start up monostable periods or switch-on transient
effects.
During the setting up procedure, the output at IC25 pin 7 is
adjusted to 0.5 volts DC at zero input current to bias the current
limit comparator pin 1 into the inhibit state.
The initial conditions are then:
______________________________________ Current limit gate 51/2 low
Negative capacitor voltage gate 52 high Positive capacitor voltage
gate 52/2 high Thyristor switch element 10 current gate 52/5 high
Thyristor switch element 7 current gate 52/12 high
______________________________________
If the firing signal (G,H) goes high, then only the second
thyristor switch element 7 can be fired via IC52 (the remaining
steering gates are inhibited). The equivalent circuit of the motor,
capacitor, and the second thyristor switch element 7 at this time
is a series L.sub.M C circuit with a series switch.
L.sub.M is the motor inductance.
C is tne commutation capacitor 9.
The circuit is under-damped with a high Q such that the voltage on
the capacitor targets for twice the supply rail, e.g., 140 volts
DC. This is clearly shown in FIG. 6 during the initial two time
divisions. It must be remembered, however, that the period of this
transition and the peak current value are influenced by the
inductance L.sub.M. This inductance can be in the range 0.2 mH to
1.6 mH. At the high inductance value, the period is longest and the
peak current at its minimum.
Should the peak current be insufficient to operate the second
thyristor switch element 7 sensor, then the second and third
thyristor switch element 7,10 inhibit and natural commutation
period cannot be ensured before the next stage of the process. A
delay period sufficient to cover this case is introduced by the
resistor 30/capacitor 31 circuit. Should the second thyristor
switcn element 7 signal be available, then capacitor 31 is reset.
This mechanism ensures that the long delay of the resistor
30/capacitor 31 circuit is only utilized at the instant of starting
from zero at the high inductance position. If this delay was
permanent, then the maximum clock rate would be unacceptably
reduced. FIG. 6 indicates the absence of the second thyristor
switch element 7 signal during the first thyristor switch element 5
firing.
Should the second thyristor switch element 7 signal be available
(it does have a minimum measurable value of approximately 10
amperes), then to ensure that sufficient time is available for the
second thyristor switch element 7 to naturally commutate and
inhibit the next stage of the commutation process, a variable delay
is imposed on the trailing edge by the resistor 56/capacitor 57
circuit ranging from 50-250 s.
As soon as the resistor 30/capacitor 31 circuit period or the
second thyristor switch element 7 signal extension is completed,
the next allowed state is to fire the third thyristor switch
element 10 to reverse the charge on the commutation capacitor. This
is clearly shown on FIG. 6 in time division 3. The resulting third
thyristor switch element 10 signal inhibits firing the first
thyristor switch element 5 of the second thyristor switch element
7. It may be observed that the next capacitor voltage transition is
positive. At the completion of the third thyristor switch element
10 current signal, the comparator 28 pin 1 has recycled the second
thyristor switch element 7 firing pulse because the negative
voltage was below 100 volts after charge reversal. The new positive
voltage is 180 volts which when charge reversed now gives -1540
volts.
This may appear a small advantage. However, there are intermediate
positive supply voltages from, for example, a low battery or a long
delay between firing pulses that can allow the charge on the
commutation capacitor to leak away via the 30 K resistor. This
internal recycling to achieve a substantial negative voltage for
principal current commutation avoids resolution problems at low
voltages. It prepares the commutation capacitor for a significant
first pulse value of main current. Once the capacitor is charged to
a suitable value of negative voltage, the first thyristor switch
element 5 is allowed to fire. The source of this firing pulse edge
is IC51 pin 13, which is directed to monostable 47 pin 5 to create
a 100 s minimum on period in which the first thyristor switch
element 5 cannot be turned off from the processor firing signal.
IC59 supervises this turn off process, which occurs if the duration
is greater than 100 s from the back edge of the processor firing
pulse.
If the process firing pulse remains high, the current in the
machine rises until the current limit system turns off the first
thyristor switcn element 5 by firing the second thyristor switch
element 7 via a differentiator resistor 60/capacitor 61 circuit at
the input of IC41 pin 9.
During the turnoff process, the commutation capacitor 9 becomes a
snubber capacitor and goes to 2 V battery and acquires the magnetic
energy of 1/2 LI.sup.2 contributed by the machine leakage
inductance (L). This positive voltage is charge reversed and the
system becomes a self synchronous chopper. This mechanism is shown
in FIG. 7.
FIG. 7 shows the operation from a low positive capacitor voltage
which is above the positive capacitor voltage sensor `low` level.
It can be seen that the capacitor voltage is reversed to an
insufficient negative level. The cycle is repeated as in the
previous text until the negative voltage is acceptable. At this
point the first thyristor 5 is allowed to be fired, then turned off
in this case by the current limit system. The new positive voltage
is charge reversed and the cycle repeated. In FIG. 7 a firing pulse
at the 15,15' logic input occurs at t=o and remains high until
t+7.4 time divisions.
The described embodiment of the present invention provides a motor
drive circuit for a motor having a bifilar winding, in which a
current sensing device aids in sequencing the condition of a
plurality of thyristor switches.
Other aspects, objects, advantages, and uses of this invention can
be obtained from a study of the drawings, the disclosure, and the
appended claims.
* * * * *